An image diagnostic apparatus using X-rays is designed to radiate X-rays generated from an X-ray generating device to an object to be examined, and detect and image a dose of X-rays which passes through the object. To generate X-rays from an X-ray tube device, DC high voltage is applied between an anode and a cathode of the X-ray tube device, and thermal electrons generated by heating the cathode to a high temperature are accelerated with DC high voltage and collided with the anode. Accordingly, a high voltage power supply for supplying the DC high voltage between the anode and the cathode is necessary.
As for this kind of X-ray high voltage device, an inverter-type high voltage device is generalized, which is greatly superior in point of device miniaturization and performance. It is currently used in almost all kinds of X-ray image diagnostic apparatus including a general X-ray imaging apparatus, an X-ray imaging apparatus for angiography, an X-ray CT apparatus, and the like.
FIG. 9 shows an example of main circuitry of the inverter-type X-ray high voltage device, in which a voltage supplied from DC power supply 1 is converted into a high-frequency AC voltage in full-bridge inverter circuit 2 having power semiconductor switching elements, e.g. insulated bipolar transistors 21, 22, 23, and 24, this AC voltage is boosted in high voltage transformer 3, converted into a DC high voltage in high voltage rectifier 4, and applied to X-ray tube 5. Primary windings of high voltage transformer 3 are formed such that two primary windings including first primary winding 3a and second primary winding 3b are connected in parallel on the output side of inverter circuit 2 in order to secure current capacity.
Further, secondary windings of high voltage transformer 3 include first secondary winding 3c and second secondary winding 3d. An output voltage of first secondary winding 3c is converted into first DC high voltage Va in first high voltage rectifier 4a and applied between anode 5a and an earth of X-ray tube 5. An output voltage of second secondary winding 3d is converted into second DC high voltage Vk in second high voltage rectifier 4b and applied between cathode 5b and the earth of X-ray tube 5. A negative side of DC voltage output terminals of first high voltage rectifier 4a and a positive side of DC output terminals of second high voltage rectifier 4k are connected in series, and the junction is grounded to the earth. This neutral grounding system is employed in the circuit.
By employing the above-described neutral grounding system, a voltage (tube voltage) between the anode and cathode of X-ray tube 5 can be divided into halves to be applied respectively between the anode and the earth and between the earth the cathode. Accordingly, it becomes easy to secure withstand voltage of the high voltage transformer and the high voltage rectifier. However, in the neutral grounding system, unbalance occurs between first DC high voltage Va and second DC high voltage Vk in some cases, and (1) and (2) listed below are the main reasons:
(1) In a glass X-ray tube and in a metal X-ray tube, difference occurs between Va and Vk due to difference between impedances of two pairs of circuits respectively for obtaining voltage Va applied between the anode and the earth and for obtaining voltage Vk applied between the earth and the cathode (impedance of a first circuit including first primary winding 3a and first secondary winding 3c and impedance of a second circuit including second primary winding 3b and second secondary winding 3d of high voltage transformer 3).
(2) In a metal X-ray tube, difference occurs between Va and Vk due to difference between load impedances respectively applied Va and Vk (impedance between anode 5a and the earth of X-ray tube 5 to which Va is applied and impedance between the earth and the cathode to which Vb is applied). Meanwhile, this phenomenon does not occur in the glass X-ray tube.
For example, in an X-ray device whose maximum tube voltage is 150 kV, the withstand voltage of secondary windings of the high voltage transformer and the voltage of an anode and a cathode to the earth of the X-ray tube can be usually estimated to be 75 kV being the half of the maximum tube voltage. However, because a voltage larger than the rating is applied between the anode and the earth or between the cathode and the earth when the above mentioned unbalance voltage occurs and becomes large, the withstand voltage not only of the X-ray tube but also of the high voltage transformer, the high voltage rectifier, and high voltage parts attaching thereto has to be set higher.
Further, an inner space called creepage distance between the high voltage parts and a housing for containing them also have to be made long in accordance with the withstand voltage. For those reasons, the apparatus is obliged to be made large when the unbalance voltage occurs, which becomes an obstacle to the above mentioned miniaturization. Particularly, it becomes a big obstacle to an X-ray CT apparatus which mounts the X-ray high voltage device on a scanner and which aims at the rapid scan or aims to reduce the number of unit of system.
Japanese unexamined patent publication No.Hei.3-101098 discloses a technique of recognizing and solving the unbalance voltage due to (2) difference in load impedance of the metal X-ray tube. This technique is designed to adjust the unbalance voltage in the metal X-ray tube of the neutral grounding system by switching a reactor of one of the plurality of primary windings of the transformer. The adjustment is done by switching the reactor with a switch while measurement is performed. Therefore, the adjustable range is stepwise and it is necessary to switch the reactor in accordance with the X-ray tube. The above adjustment cannot be performed in the X-ray device on which this X-ray generating device is mounted while the tube voltage is actually applied to the X-ray tube to perform imaging. Accordingly, the adjustment had to be done regularly.